1. Field of the Invention
The present invention relates to a silicon carbide single crystal which can be suitably used as a semi-insulating or insulating single crystal substrate and the like, and an effective method of producing the same. Further, the present invention relates to a silicon carbide single crystal which can be suitably used as a p-type semiconductor and the like, and an effective method of producing the same. The present invention relates to a seed crystal fixing apparatus for fixing a seed crystal to a seed crystal setting part of a reaction vessel with interposition of an adhesive, and a method for fixing the seed crystal.
2. Related Art
Silicon carbides have larger band gap and more excellent in dielectric breakdown property, heat resistance, radiation resistance and the like as compared with silicon, therefore, have been noticed as electronic device materials such as portable and high output semiconductors and the like, and due to excellent optical properties, noticed as optical device materials. Among such silicon carbide crystals, silicon carbide single crystals have a merit that they are particularly excellent in uniformity of properties in wafer when applied to devices such as wafers and the like as compared with silicon carbide polycrystals.
As a method of producing the above-mentioned silicon carbide single crystal, Improved Rayleigh method (improved sublimation re-crystallization method) is known in which a graphite crucible is used, a silicon carbide powder is sublimated and a silicon carbide single crystal is grown on a seed crystal of a silicon carbide single crystal, however, there are known few methods of producing a silicon carbide single crystal having a content of impurity elements (elements of atomic number of 3 or more belonging to group I to group XVII elements in the periodic table of 1989 IUPAC inorganic chemical nomenclature revision (excluding, a carbon atom, nitrogen atom, oxygen atom and silicon atom), hereinafter the same) of 1.0 mass ppm or less.
On the other hand, a silicon carbide single crystal obtained by using a silicon carbide powder containing a significant amount of nitrogen atoms not included in the above-mentioned impurity elements cannot be utilized as a semi-insulator or insulator though it can be utilized as an n-type semiconductor, since nitrogen imparts, as a donor atom, electron conductivity to a silicon carbide crystal. Consequently, in some devices such as MESFET and the like using high frequency, use of high insulating substrates is desired for the purpose of suppression of high frequency loss of a substrate and the like, however, under current conditions, a method of producing a silicon carbide single crystal suitable for such a high insulating substrate has not been provided yet.
As a method of producing a silicon carbide single crystal which can be utilized as a p-type semiconductor, a method is known in which a powder of aluminum or alumina is added to a sublimation raw material silicon carbide powder and the mixture is sublimated simultaneously to produce a silicon carbide single crystal. In the case of this method, an aluminum atom in the grown silicon carbide single crystal supplies as an acceptor a hole, to manifest p-type electric conductivity. However, in this case, when nitrogen is contained in a sublimation raw material silicon carbide powder, there is a problem that compensation occurs between a nitrogen atom acting as a donor and an aluminum atom acting as an acceptor, resulting in loss of conductivity. There is no method provided under current conditions which can effectively prevent such compensation and can efficiently produce a silicon carbide single crystal suitable as a p-type semiconductor.
When the seed crystal is grown without being completely bonded to the seed crystal setting part in improved Lely method, macroscopic defects (void defects) penetrate through the seed crystal from the side of the seed crystal setting part where the seed crystal is not completely bonded into the growing crystal, and the quality of the wafer was liable to be impaired. Since the adhesive is gasified at high temperatures, it has been also conjectured that remaining bubbles of the gasified adhesive in the adhesive layer causes deterioration of the quality of the crystal.
Several techniques have been proposed for solving the above-mentioned problems (for example. see Japanese Patent Application Laid-Open Nos. 2001-139394 and 2003-119098). Japanese Patent Application Laid-Open No. 2001-139394 discloses, for example, a method for fixing the seed crystal by applying a given pressure. Japanese Patent Application Laid-Open No. 2003-119098 discloses a fixing method for press-bonding the seed crystal by mounting a weight on the seed crystal.
However, it was difficult to uniformly bond the entire surface of the seed crystal by the mechanical press-bond method which cause the surface of the seed crystal finely roughened. Uniform press-bonding with a weight was also difficult.